Molecular basis of F-actin regulation and sarcomere assembly via myotilin.

• Main Authors: Julius Kostan, Miha Pavšič, Vid Puž, Thomas C Schwarz, Friedel Drepper, Sibylle Molt, Melissa Ann Graewert, Claudia Schreiner, Sara Sajko, Peter F M van der Ven, Adekunle Onipe, Dmitri I Svergun, Bettina Warscheid, Robert Konrat, Dieter O Fürst, Brigita Lenarčič, Kristina Djinović-Carugo
• Format: Article
• Language: English
• Published: Public Library of Science (PLoS) 2021-04-01
• Series: PLoS Biology
• Online Access: https://doi.org/10.1371/journal.pbio.3001148

Sarcomeres, the basic contractile units of striated muscle cells, contain arrays of thin (actin) and thick (myosin) filaments that slide past each other during contraction. The Ig-like domain-containing protein myotilin provides structural integrity to Z-discs-the boundaries between adjacent sarcomeres. Myotilin binds to Z-disc components, including F-actin and α-actinin-2, but the molecular mechanism of binding and implications of these interactions on Z-disc integrity are still elusive. To illuminate them, we used a combination of small-angle X-ray scattering, cross-linking mass spectrometry, and biochemical and molecular biophysics approaches. We discovered that myotilin displays conformational ensembles in solution. We generated a structural model of the F-actin:myotilin complex that revealed how myotilin interacts with and stabilizes F-actin via its Ig-like domains and flanking regions. Mutant myotilin designed with impaired F-actin binding showed increased dynamics in cells. Structural analyses and competition assays uncovered that myotilin displaces tropomyosin from F-actin. Our findings suggest a novel role of myotilin as a co-organizer of Z-disc assembly and advance our mechanistic understanding of myotilin's structural role in Z-discs.